Antennas for radio base stations are physically large compared to other radio components, but typically have relatively wider bandwidth. Therefore, it is preferable to have multiple transmit (TX) bands and receive (RX) bands share an antenna so that the number of antennas can be minimized.
In order to share an antenna a multiplexer is typically used. FIG. 1 shows a conventional 4-band multiplexer 10 that couples two transmit signals and two receive signals to an antenna 12. Each of the transmit signals and the receive signals pass through a respective filter 14, 16. For high-power base stations, air cavity filters having low passive intermodulation (PIM) may be employed to implement the multiplexer 10. However, for low-power base stations transmitting and receiving in small cells, these air cavity filters are too large and miniature filters are preferred. Such miniature filters are typically acoustic filters such as surface acoustic wave (SAW), bulk acoustic wave (BAW) and thin-film bulk acoustic resonator (FBAR) filters, or are ceramic filters. However, these types of filters have limited PIM performance, even at low power. Thus, as shown in FIG. 1, the transmitter filters 14 undesirably end up feeding PIM to receive circuits via the receive filters 16, thereby reducing receiver sensitivity.
FIG. 2 shows a type of 4-band multiplexer that exhibits low PIM. In FIG. 2, two transmit signals, TX1 and TX2, are combined by a combiner 18 and input to port 7 of the lower 3 dB 90 degree hybrid coupler 20. The combined signal travels via two different paths through the lower hybrid coupler 20. A first path is from port 7 to port 5 (the direct port), and a second path is from port 7 to port 6 (the coupled port). The signal from port 5 passes through a dual band TX band pass filter 22 and is input to port 4 of the upper 3 dB 90 degree hybrid coupler 24. The signal from port 6 passes through a dual band TX band pass filter 26 and is input to port 3 of the upper hybrid coupler 24. The signal arriving at port 4 from the TX band pass filter 22 travels via two different paths through the upper hybrid coupler 24. The first path is from port 4 to port 1 to an antenna 28. The second path is from port 4 to port 2. Similarly, the signal arriving at port 3 of the upper hybrid coupler 24 travels via two paths. The first path is from port 3 to port 1 and to the antenna 28. The second path is from port 3 to port 2. The signals from port 2 are passed through a dual band RX band reject filter (BRF) 30 and split into receive signals RX1 and RX2 by a 2-band splitter 32.
Ideally, the two TX BPFs are identical. Also, the signal travelling from port 7 to port 5, i.e., the direct port, has a 90 degree phase difference with the signal travelling from port 7 to port 6, i.e., the coupled port. Similarly, in the upper hybrid coupler 24, for the input at port 4, the output at the direct port 2 has a 90 degree phase difference with the signal at the coupled port 1. Thus, the signal from port 7 to 5 to the TX BPF 22 to port 4 to 2 will ideally be 180 degrees out of phase with the signal from port 7 to 6 to the TX BPF 26 to port 3 to 2. Thus, the signals of these two paths will cancel each other. Conversely, the two signals that reach port 1 will add constructively since they are in phase.
A signal received by the antenna 28 at port 1 will be split in two signals, one traversing from port 1 to 4 (the coupled port) and one traversing from port 1 to port 3 (the direct port). The signal arriving at port 4 will be reflected due to the receive band rejection of the TX BPF 22. The reflected signal will be reflected back to port 1 and will also be reflected to port 2. Similarly, the signal arriving at port 3 will be reflected due to the receive band rejection of the TX BPF 26. This reflected signal will be reflected back to port 1 and will also be reflected to port 2. The signal that traverses from port 1 to port 4 to port 2 will add constructively at port 2 with the signal that traverses from port 1 to port 3 to port 2. Conversely, the two signals reflected back to port 1 will destructively interfere and cancel.
The above discussion assumes that the hybrid couplers provide a 90 degree phase shift and equal splitting of the signal power over the entire relevant frequency band. Further, the above discussion assumes a flat amplitude frequency response that is the same for the path to direct port and a path to a coupled port. In reality, the hybrid couplers may have a frequency response as shown in FIG. 3. As shown in FIG. 3 the frequency response 34 of the coupler direct port is concave down, whereas the frequency response 36 of the coupler coupled port is concave up. At the lower frequency of the lower transmit band TX1, the difference between the coupling between the direct port and the coupled port is about 0.35 dB. Thus, if the lower and upper hybrid couplers 20 and 24 have the response shown in FIG. 3, the signal that traverses from port 7 to 6 to 3 to 2 at frequency F1 would have power about 0.7 dB higher than the power of the signal that traverses from port 7 to 5 to 4 to 2. This results in leakage of the TX1 band signal near frequency F1 of about 10 dB into the receive path. At the same time, this also results in an insertion loss of the path from port 7 to port 1 of about 0.1 to 0.3 dB. Further, as shown in FIG. 3, the output power difference between the coupled port and the direct port gets bigger as the frequency decreases. This means that PIM generated by the dual band TX BPFs 22, 26 will be exacerbated when the receive band is on the lower side of the transmit band, which is often the case. Further, since a dual band BPF has a higher insertion loss than a single band BPF, the multiplexer configuration of FIG. 2 has a high insertion loss.
For low power radio base stations, resonator-type ceramic filters, such as coaxial ceramic filters, monoblock filters, and ceramic waveguide filters, etc., are sometimes used for RF filtering of the received signal. However, for some applications, ceramic filters are too bulky and too expensive. Miniature SAW, BAW or FBAR filters have therefore been proposed for use. FIG. 4 shows an example of such proposal. FIG. 4 shows an antenna 38, a first acoustic filter 40, a low noise amplifier 42, and a second acoustic filter 44. These filters 40 and 44 together in series show a large amplitude variation in the pass band of their frequency response, as shown in FIG. 5. FIG. 5 shows a typical pass band frequency response of the circuit of FIG. 4. The pass band variation is greater than 2 dB. Further, insertion loss is highest at the pass band edges and lowest at around the center of the pass band. Further, the configuration of FIG. 4 does not provide sufficient out of band rejections.
As shown in FIG. 6, the total receive front end out-of-band filtering performance 46 of the configuration of FIG. 4 does not meet the rejection filter rejection envelope requirements 48 set out in known wireless communication standards. This deficiency is common to all acoustic wave filters. The problem of poor out-of-band rejection cannot be resolved by changing the acoustic filter design. In general, a single acoustic filter can only provide 10 to 20 dB of rejection at above-band frequencies. Two such filters can be expected to achieve only 20 to 40 dB of rejection at above-band frequencies.